How to get the date of next specified day of week

问题

I have to calculate the next date based on the current date. For example, say today is '05-NOV-2014' and user have gave an input like 1st Monday. so based on the user input if the 1st Monday of the current month has already gone past then i have to find out the 1st Monday of next month. if the 1st Monday is yet to come in the current month then i have to find out the date within the current month.

I am using java.util.Calendar class.

//user inputs
int dayOfWeek = 3; // as user has provided 'Tuesday'   
int noOfWeek = 1; // as user provided 1st 

Calendar cal = Calendar.getInstance(); // getting the current instance.
int currentDayOfWeek = Calendar.getInstance().get(Calendar.DAY_OF_WEEK); // getting the cCurrentDayOfWeek in integer.
int currentNoOfWeek  = Calendar.getInstance().get(Calendar.DAY_OF_WEEK_IN_MONTH); // getting which week it is in the current month


 if(noOfWeek < currentNoOfWeek){
 // the date has gone past in the current month.
 }
 else if ((noOfWeek == currentNoOfWeek) && dayOfWeek < currentDayOfWeek){
 // though the week number is same but as the currentDayOfWeek is greater than the provided day so in this case also  date has gone past in the current month.
 }

No able to proceed further. seeking your help.

回答1:

It was a pretty cool problem you have, and this is the solution I came up with and my approach:

First what you had:

public static void main(String[] args) {
    // TODO: validate user-input

    // Input by user:
    int inputDayOfWeek = 3; // Tuesday
    int inputWeekOfMonth = 2;

    if(isInNextMonth(inputDayOfWeek, inputWeekOfMonth)){
        Date outputDate = calculateNextValidDate(inputDayOfWeek, inputWeekOfMonth);

        // Do something with the outputDate
        System.out.println(outputDate.toString());
    }
}

private static boolean isInNextMonth(int inputDayOfWeek, int inputWeekOfMonth){
    // Current day:
    Calendar cal = Calendar.getInstance();
    int currentDayOfWeek = cal.get(Calendar.DAY_OF_WEEK);
    int currentWeekOfMonth = cal.get(Calendar.DAY_OF_WEEK_IN_MONTH);

    // The date has gone past in the current month
    // OR though it's the same week of the month, the day of the week is past the current day of the week
    return inputWeekOfMonth < currentWeekOfMonth || ((inputWeekOfMonth == currentWeekOfMonth) && inputDayOfWeek < currentDayOfWeek);
}

Some things to note: I've put both the if and if-else into a single if, since in both cases you want to go to the next month, and also made it a separate method (making it a separate method is just a preference of myself, to keep things structured and organized).

Another thing I've noticed was an error in your if and else-if. It should be noOfWeek < currentNoOfWeek instead of noOfWeek > currentNoOfWeek and ((noOfWeek == currentNoOfWeek) && dayOfWeek > currentDayOfWeek) instead of ((noOfWeek == currentNoOfWeek) && dayOfWeek < currentDayOfWeek) (the < and > are reversed).

Now the calculateNextValidDate-method, which was where your problem lies. My approach is as follows:

Start on the first day of the next month
Go to the correct week of this month
Then go to the correct day of this week

This gave me the following code:

private static Date calculateNextValidDate(int inputDayOfWeek, int inputWeekOfMonth){
    // Set the first day of the next month as starting position:
    Calendar cal = Calendar.getInstance();
    cal.add(Calendar.MONTH, 1);
    cal.set(Calendar.DAY_OF_MONTH, 1);

    // Now first go to the correct week of this month
    int weekOfNextMonth = 1;
    while(weekOfNextMonth < inputWeekOfMonth){
        // Raise by a week
        cal.add(Calendar.DAY_OF_MONTH, 7);
        weekOfNextMonth++;
    }

    // Now that we have the correct week of this month,
    // we get the correct day
    while(cal.get(Calendar.DAY_OF_WEEK) != inputDayOfWeek){
        // Raise by a day
        cal.add(Calendar.DAY_OF_MONTH, 1);
    }

    return cal.getTime();
}

This code gave me the following output (on Wednesday 5th of November 2014 - with 3 [Tuesday] and 2 as input):

Tue Dec 09 17:05:42 CET 2014

Also note the // TODO: I've added in the main-method of the first code-part of this post. If the user-input is invalid (like a negative week or dayOfMonth for example), it can go through the while-loops too many times. I leave it up to you to validate the user-input.

回答2:

Here's an idea. Continue to roll your Calendar instance one day [cal.add(DAY, 1)] per iteration (i.e., wrap in loop), each time checking that you are in the Day/Week specified as input. Finish and extract the date once such condition is met.

Also, substitute calls to Calendar.getInstance().get() for cal.get() after the "cal" variable declaration.

来源：https://stackoverflow.com/questions/26754164/how-to-get-the-date-of-next-specified-day-of-week

标签

java

date

java.util.calendar